1. Field of the Invention
The present invention relates generally to a digital power amplifier and pertains, more particularly to a digital power amplifier which utilizes a high frequency linear modulated driving signal. Even more particularly, the invention relates to a digital power amplifier which inputs a high frequency modulated linear signal to the drivers of a bridge circuit to obtain a balanced output.
2. Background Discussion
There are many different types of power amplifiers. Most power amplifiers are of the linear or analog type. Linear amplifiers fall into one of the following classifications: A, AB, B and C. These linear amplifiers provide the function of increasing and amplifying both voltage and power capabilities of a circuit. Due to the fact that these amplifiers operate in the linear region, they have low efficiencies and dissipate a considerable amount of heat. The following table lists the approximate efficiency for each class of amplifier:
______________________________________ Type Efficiency ______________________________________ Class A 20% Class AB 40% Class B 40%-60% Class C 60%-80% ______________________________________
The distortion increases with increasing efficiency which means that one has to sacrifice distortion for efficiency or visa versa. In addition, the efficiency of the linear power amplifier is reduced when a coupling transformer is used to drive an output load, which is common.
Digital amplifiers are more efficient than linear or analog amplifiers. Class D digital amplifiers are well-known in the art and prior art Class D digital amplifiers employ pulse width-modulation techniques. The circuits employed utilize a fixed ramp or sawtooth waveform for providing a threshold voltage level as one input of a comparator. The audio or other signal, to be amplified, is connected to the other input of the comparator. The output of the comparator is switched when the input signal exceeds the sawtooth threshold signal. While more efficient than linear amplifiers, digital amplifiers tend to be very complex and expensive. In addition, component or inherent signal variations of the circuit may cause efficiency and distortion problems.
In prior art digital amplifiers, such as those used for motor control, digital signals are used to drive the bridge circuit or equivalent portion of the amplifier. Digital signals are used as inputs to driving comparators. This causes problems due to the uncertainty of the threshold level of the comparator, as well as the complexity and relative lack of control that the digital signal provides. A digital signal is either on or off. When a digital signal is applied as an input to a bridge circuit arrangement of driving comparators, which in turn drive a bridge circuit of transistors, no dead band exists and cross conduction between complimentary transistor pairs results. When such cross conduction occurs between complimentary transistor pairs in a bridge circuit, an unbalanced, uncontrollable output results. Prior art embodiments have attempted to resolve this phenomenon by using circuit combinations including delay circuits, flip flops and comparators; a complex, expensive, and unreliable solution.
Accordingly, it is an object of the present invention to provide a more simplified and inexpensive power amplifier which operates at extremely high efficiencies.
Another object of the present invention is to provide a power amplifier which, while operating at extremely high frequencies, dissipates very little heat and operates with extremely low distortion.
Another object of the present invention is to provide a digital power amplifier which transmits analog signals and provides a means for adjusting the output waveform to compensate for component or inherent signal variations of the circuit.
Another object of the present invention is to provide a digital power amplifier which need not require using a digital signal as the input to the driver of a bridge circuit.
Another object of the present invention is to provide a digital power amplifier which utilizes a bridge circuit arrangement of transistors, in which a dead band exists with no cross conduction between the complimentary transistor pairs.
A final object of the present invention is to provide a power amplifier which avoids using complex circuity while very simply providing an infinitely controlled dead band to suit the characteristics of the bridge circuit